1. Field of the Invention
The present invention relates to a vertical furnace used in the fabrication of semiconductor devices, and more particularly, to a single bodied cap for a vertical furnace which supports a semiconductor wafer boat and maintains an appropriate temperature in the vertical furnace.
2. Description of the Related Art
Various films, such as polysilicon, oxide, and nitride films, are formed in a furnace in order to manufacture semiconductor devices. The furnace should be extremely stable and have a high degree durability since combustible poisonous gases are used at high temperatures to form the various films therein. The furnace should also prohibit the generation of contaminating particles, and the parts of the furnace should be easily changeable after installation of the furnace.
In particular, since the cap for supporting the boat on which wafers are loaded is also the portion through which reactive gases are initially passed, the cap itself should not contain any contaminating particles, should be durable, and should be easy to assemble and replace.
FIG. 1 is a perspective view of a furnace in which a conventional cap is used. FIG. 2 is an exploded perspective view of the main portions of the furnace of FIG. 1. Referring to FIGS. 1 and 2, the furnace comprises a cap 12, a holder 18, and a base 20 on which the cap 12 is mounted. A boat 10, into which wafers 11 are placed, is composed of two parallel circular plates 10b and 10c connected by a plurality of supporting rods 10a, and two connection holes 10d formed in the lower circular plate 10c for connecting the boat 10 to the cap 12.
The cap 12 supports the boat 10 and has a cylindrical structure having an open bottom. At the upper surface of the cap 12, screw holes 12a are formed, corresponding to the connection holes 10d of the lower circular plate 10c. The boat 10 and the cap 12 are connected via screws 14 through the connection holes 10d and screw holes 12a.
The holder 18, within which a plurality of adiabatic plates 16 are stacked side by side, has a similar structure to that of the boat 10 and fits inside the cap 12 after the adiabatic plates 16 have been placed therein. A through hole 18b for connecting the holder to the base 20 is formed in the lower circular plate 18a of the holder 18. The adiabatic plates 16 maintain a uniform temperature inside the heat treatment tube 24.
The base 20 comprises a flanged supporting portion 20b, a projecting portion 20a at the center of the flanged supporting portion 20b for connecting the base 20 to the holder 18, and a tube supporting portion 20c, having a diameter larger than that of the heat treatment tube 24, to support the heat treatment tube 24 placed thereon.
The tube supporting portion is connected to an elevating means 22. The elevating means 22 is, for example, a hydraulic jack, for raising the boat 10 into the heat treatment tube 24.
In the conventional vertical furnace described above, the means for supporting the boat 10 and maintaining an appropriate temperature for the reaction of gases comprises three main parts, i.e., the cap 12, the holder 18, and the adiabatic plates 16. Since the respective parts are formed of quartz, however, the friction generated among the respective parts during the assembly process may cause a crack or destroy any one of the parts. In addition, the fine quartz powder generated during the assembly process may become a contaminant during subsequent diffusion or deposition manufacturing processes.
Another disadvantage is that since the cap 12 is tightly closed, some of the reactive gases supplied to the inside of the heat treatment tube 24 rise along the narrow spaces between the cap 12 and the heat treatment tube 24 and reach the lower portion of the boat 10, forming an open space. Other reactive gases that rise rapidly flow to the upper portion of the boat 10, forming a vortex by spreading rapidly into the open space of the lower portion of the boat.
Thus, the flow of the reactive gases on the surfaces of the wafers is not uniform and will vary depending on the respective positions of the wafers inside the boat 10. Also, non-activated gases can act as contaminating particles on the wafers. Such problems are more severe at the lower portions of the boat.
Moreover, there are many problems in maintaining the parts of the furnace since the durability of the respective parts will vary and the procedures for replacing the parts are complicated.